perf jevents: Add legacy json terms and default_core event table helper

perf jevents: Add legacy json terms and default_core event table helper

Add json LegacyConfigCode and LegacyCacheCode values that translate to
legacy-hardware-config and legacy-cache-config event terms
respectively.

Add perf_pmu__default_core_events_table as a means to find a
default_core event table that will later contain legacy events.

In situations like hypervisors it is more likely that tables will be
NULL. Rather than testing in the calling PMU code, early exit in the
pmu-event.c routines.

Tested-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>